Production of cyclohexadiene dicarboxylic acids



United States Patent Int. Cl. C07c 31/40, 61/16 US. Cl. 204-73 ClaimsABSTRACT OF THE DISCLOSURE A process for the manufacture ofcyclohexadiene dicarboxylic acids by partial hydrogenation of thecorresponding phthalic acids. The process is carried outelectrochemically in dilute aqueous sulfuric acid solution at atemperature of more than 70 C. at a current density of 1 to 40 amps persq. decimeter or a cathode consisting of:

(a) either pure cadmium, tin or bismuth, or

(b) a solid alloy of at least two of the metals lead,

mercury, silver, cadmium, tin, thallium and bismuth,

(c) of amalgamated lead, amalgamated thallium, an

amalgamated metal of group (a) or an amalgmted solid alloy of group (b).Cyclohexadiene carboxylic acids are suitable for the production ofcyclic polycarboxylic acids, e.g., by reaction with maleic acid, whichare excellent plasticizers, e.g., for polyvinyl chloride.

The present invention relates to a process for the production ofcyclohexadiene dicarboxylic acids, more particularly to the use ofcertain cathodes in the electrochemical production of said carboxylicacids from the corresponding phthalic acids.

It is known that 3,S-cyclohexadiene-1,2-dicarboxylic acid can beprepared by electrochemically hydrogenating o-phthalic acid with dilutesulfuric acid as the catholyte above 60 C. For example, according toBerichte der Deutschen Chemischen Gesellschaft, vol. 39 (1906), pages2933 to 2942, and Zeitschrift fiir Electrochemie, vol. 35 (1929), pages769 to 779, the reaction is carried out on specially prepared, very purelead cathodes in 15% sulfuric acid. In US. patent specifications2,477,579 and 2,477,580 the disadvantages of the lead cathodes aredescribed and the use of mercury cathodes in 5% sulfuric acidrecommended. Furthermore, apparatus is described in US. patentspecification 2,537,304 whose purpose is to prevent poisoning occurring.When lead cathodes are used, poisoning occurs and this considerablyreduces the reactivity of the cathodes. Moreover, brown tar-likebyproducts are formed which discolor the reaction product andnecessitate a special purification process. Even when mercury is used,poisoning of the cathodes still occurs, although not until the reactionhas been in progress for some time. It is therefore necessary tocontinuously remove the mercury from the hydrogenation cell, purify it3,471,381 Patented Oct. 7, 1969 and return the purified mercury to thecell. The mercury must therefore be recycled in a separate loop whichincludes at least one purification stage. In this way the cathodesreactivity is maintained but the method required to carry out theelectrochemical process over a long period is expensive. Moreover,special saftey precautions must be observed because of the healthhazards involved in using mercury and in particular in operating amercury oop.

It is an object of the invention to provide a new, more effectiveprocess for the manufacture of cyclohexadiene dicarboxylic acids fromthe corresponding phthalic acids. It is another object to providecathodes for the electrochemical partial hydrogenation of phthalicacids. It is a further object of the invention to provide a process forthe manufacture of cyclohexadiene dicarboxylic acids in which longon-stream period can be achieved. A still further object of theinvention is to provide a simple process for the manufacture ofcyclohexadiene carboxylic acids in which complicated system for mercuryrecycling is not needed. These and other objects will be betterunderstood from the following detailed description.

We have found that 3,5-cyclohexadiene-1,2-dicarboxylic acid or2,5-cycloheXadiene-1,4-dicarboxylic acid may be obtained by theelectrochemical hydrogenation of ophthalic acid or terephthalic acid indilute aqueous sulfuric acid at temperatures above C. and with a currentdensity of 1 to 40 amps per sq. decimeter, if desired using lead ormercury cathodes, without the difliculties usually encountered with leadand mercury cathodes, when the cathode used is amalgamated lead,cadmium, tin, thallium or bismuth, or alloys, which may also beamalgamated, of at least two of the metals lead, mercury, silver,cadmium, tin, thallium and bismuth or pure cadmium, tin or bismuth.

Even when the cathodes according to this invention are used incontinuous operation no poisoning is observed. In the course of time thecathode potential is negativated which becomes apparent from theformation of small amounts of hydrogen; said formation of hydrogen,however, improves convection and advantageously changes theconcentration of the phthalic acid to be hydrogenated or of thehydrogenation product at the cathode. The advantages of the new processare obvious: in contrast to processes where lead cathodes are used nodisturbances occur, and in contrast to processes where mercury cathodesare used a considerably simpler method of operation is possible.

By amalgamation we mean the coating of metals or alloys with a mercuryalloy by applying small amounts of mercury to the metal or alloy.

The amalgamated cathodes which may be used are of lead coated with analloy of lead and mercury, cadmium coated with an alloy of cadmium andmercury, tin coated with an alloy of tin and mercury, thallium coatedwith an alloy of thallium and mercury or bismuth coated with an alloy ofbismuth and mercury. The binary, ternary or higher alloys which may beused are alloys of lead and mercury; lead and silver; lead and cadmium;lead and tin; lead and bismuth; lead and thallium; mercury and silver;mercury and cadmium; mercury and tin; mercury and bismuth; mercury andthallium; cadmium and tin; cadmium and bismuth; cadmium and thallium;tin and bismuth; tin and thallium; bismuth and thallium; lead, mercuryand silver; lead, mercury and cadmium; lead, mercury and tin; lead,mercury and bismuth; lead, mercury and thallium; lead, silver andcadmium; lead, silver and tin; lead, silver and bismuth; lead, silverand thallium; lead, cadmium and tin; lead, cadmium and bismuth; lead,cadmium and thallium; lead, tin and bismuth; lead, tin and thallium;lead, thallium and bismuth; mercury, silver and cadmium; mercury, silverand tin; mercury, silver and bismuth; mercury, silver and thallium;mercury, cadmium and tin; mercury, thallium and bismuth; mercury,cadmium and bismuth; mercury, cadmium and thallium; mercury, tin andbismuth; mercury, tin and thallium; silver, cadmium and tin; silver,cadmium and bismuth; silver, cadmium and thallium; silver, tin andbismuth; silver, tin and thallium; silver, thallium and bismuth;cadmium, tin and bismuth; cadmium, tin and thallium; cadmium, thalliumand bismuth; and tin, thallium and bismuth; examples of higher, e.g.,quaternary, alloys are alloys of lead, mercury, silver and tin; lead,mercury, silver and thallium; and lead, thallium, cadmium and tin, allof which may be amalgamated.

The relative proportions of the individual metals may vary within a widerange, the limits being determined by the mechanical processability ofthe resulting alloy. In the case of cadmium, tin and bismuth pure, i.e.100%, metals may be used. When mercury is used as amalgam or in analloy, the content of mercury should be chosen in such a way that themercury alloy or the amalgamated metal remains solid at the reactiontemperature. The limits with leadzmercury are, therefore, from 30:70 to99.5:0.5 or 0.1 to 2 g. of mercury per square decimeter of lead in theamalgamation, or, for example, with thallium:mercury the limits are from50:50 to 99.9:0.l, with lead: mercury:silver from 30:65.9:05 to99.5:0.5: to 3010.5: 69.5 parts by weight. The proportions are similarwith other mercury-containing cathodes. It is preferred to use cadmium,cadmium:tin=100:0 to 50:50, leadzmercury: 50:50 to 90:10,leadzmercuryzsilver=50z30 to 90:9.5: 0.5, leadzbismuth=30c70 to 80:20,lead:tin=20:80 to 80.20, lead:cadmium=0.5:99.5 to 80:20,cadmium:bismuth=100z0 to 20:80 are used, all the cathodes with andwithout amalgamation, as well as amalgamated lead. Lead or platinummetal is usually used for the anode. Lead dioxide, graphite, siliconcarbide and other substances which are resistant, or at leastsubstantially, resistant to anodic dissolution.

Electrolysis is carried out in conventional electrolytic cells, in whichthe anode and cathode are separated by a diaphragm, for example ofporous clay. The anode and cathode are arranged in conventional manner,for example a double-Walled tube may be used which may be heated orcooled and in which the cylindrical cathode is arranged on the innerwall of the tubular cell or is the inner wall of the tube and surroundsthe likewise cylindrical diaphragm. The anode is for example, awater-cooled lead pipe. However, throughlike cells with electrodes inthe form of plates and with suitable cooling elements may be usedequally Well.

The process may be used for the partial hydrogenation of both o-phthalicacid, which may also be in the form of phthalic anhydride, andterephthalic acid. The particular phthalic acid may be dispersed in aconcentration of 2 to 8%, preferably 3 to 6%, by weight, in diluteaqueous sulfuric acid which generally contains more than 2% but not morethan 50%, preferably 3 to 20%, by weight of sulfuric acid. Attemperatures above 70 C. a sufiiciently large amount of the particularphthalic acid goes into solution for the partial hydrogenation toproceed smoothly. It is advisable to exploit the maximum solubility ofthe phthalic acid at the particular reaction temperature and with thegiven sulfuric acid content. The values can be easily determined bysimple experiments, e.g., 4.95 g. of o-phthalic acid dissolves in 100 g.of 5% sulfuric acid at 85 C. In general it is not necessary to usetemperatures above 100 C. It is preferably to carry out the process attemperatures between and 98 C. Dilute sulfuric acid is advantageouslyused as the anolyte as wellin approximately the same concentration as inthe case of the catholyte. The concentration may, however, be slightlyhigher.

Electrolysis may be carried out with a current density of 140,preferably 3-20, amps/sq. decimeter (cathode current). In a preferred,continuous method of operation the catholyte is fed to the cathodechamber of the electrolytic cell by means of a metering pump, themixture of phthalic acid and sulfuric acid being allowed to flow fromthe bottom to the top of the cell. The hydrogen formed providesadditional convection. After residence times of for example 0.1 to 5hours in the electrolytic cell the reaction mixture is cooled,preferably to below 15 C. The cyclohexadiene dicarboxylic acid which hasbeen formed then crystallizes out. It is separated in conventionalmanner and the mother liquor, after phthalic acid has been added to it,can be returned to the electrolytic cell.

After being simply washed with water the cyclohexadiene dicarboxylicacids prepared in this way are obtained in extremely pure form. Sincethe cyclohexadiene dicarboxylic acids are very reactive on account oftheir double bonds, it is recommendable to work them up at relativelylow temperatures, preferably below 20 C.

The process according to the present invention is further illustrated bythe following examples.

EXAMPLE 1 The hydrogenation cell consists of a double-walled glass tubeprovided with a cylindrical amalgamated lead cathode, a cylindrical claydiaphragm and a water-cooled lead tube as anode. The catholyte isintroduced at the bottom of the cell by means of a metering pump andwithdrawn at the top.

Height of cell 57 cm.

Diameter of cell 6.5 cm.

Cathode area 10 sq. decimeters.

Anode area 3.1 sq. decimeters.

Cathode chamber 1 liter.

Reaction temperature C.

Current density l0 amps./dm.

Applied current 150% of theoretical.

Catholyte 5% H SO +4.5%

o-phthalic acid.

Anolyte 5% H 50 Catholyte throughput 4.6 kg./h. (206.5 g.

of o-phthalic acid).

o-Phthalic acid conversion 3,5-cyclohexadiene-l,2-

dicarboxylic acid 88% of the theory. Space-time yield 0.184 kg. of 3,5-cyclohexadiene 1,2-

dicarboxylic acid per reaction space per hour.

Current efficiency 58.75%.

In sustained operation over a period of eight days there was no changein the result ownig to poisoning of the cathode. Analysis of thereaction product showed no traces of 2,6-cyclohexadiene dicarboxylicacid. There were no tar-like impurities.

EXAMPLES 2-26 The following results were obtained using various cathodesand operating under the same conditions as described in Example 1.

Yield of 3,5-cyclodihexslaene- 2-dicar 1O boxylic o-Phthallc acid,Current acid yield, percent efliciency, Ex. Cathode percent of theorypercent 2 Pb:Hg=75:25 amalga- 97 90 58. 7

mated. 15 3 Pb:Hg=75:25 not amal- 96 91 59 gamated. 4 Ph:Hg:Ag=80:19:1not 99 86 57.6

amalgamate 5 OdzSn=90z10 not 100 87 56.8

84 79 53 87 81 54. 5 2O 99 91 58. 5 T1 as a 22-: T1 amalgamated. 11'II:Hg=85:15- 97.5 90. 5 57 12- Tl:Pb=80:20 96. 5 89. 5 56 13-Pb-Cd=50-50. 99 90.5 55.5 14. CdzBi=5O 50 100 89 56.8 25 15- Sn'B1=5050-- 86. 5 83.5 54 16 Pb.Sn=36.64 98 88 55 17 PbzBi=50z50 99 88.5 56 18Pb:Bi50:50 amalga- 100 90 56. 5

mate 19 PbzAggQOzlO amalga- 99 89. 5 56. 5 30 mate 20 AgzCd=10z90 10090. 5 58 21 Pb:Cd:Ag=t5%:40:1O 99 89.3 57.2

amalgarna e 22 Bi:Sn:Hg=40=50:11 98 88.7 56.1 23 Cd:Tl:Ag=80:10:10 10088.9 57. 5 24 Pb:Cd:B1:Hg=40:30: 98 87.8 55.8

:5. 25 Sn:Tl:Cd:Ag=:10: 97 87.5 66.8

51:10. 26 Pb:Sn:T1:Cd=40:20:20: 97.5 88.1 56.3

20 amalgamated.

EXAMPLE 27 120 cc. of a suspension of very finely ground terep hthalicacid (10 g.) in 5% sulfuric acid is hydrogenated at a temperature of toC. in a cylindrical glass electrolytic vessel containing a trough-likeclay diaphgram surrounded by a cylindrical cathode of an amalgamatedlead/mercury allow (90% Pb+10% Hg). 15% sulfuric acid is used as theanolyte. The anode is of platinum and the current density is 5 amps persq. decimeter. The reaction is stopped after one hour, the suspension iscooled to 15 C. and the precipitate is filtered and dried. NMR analysisshows a yield of 89% of the theory of2,5-cyclohexadiene-1,4-dicarboxylic acid with reference to the reactedterephthalic acid.

We claim:

1. A process for the production of 3,5-cyclohexeadiene-1,2-dicarboxylicacid or 2,5-cyclohexadiene-1,4-dicarboxylic acid which compriseselectrochemical hydrogenation of o-phthalic acid or terephthalic acid inaqueous sulfuric acid at temperatures of from 70 to C. and with acurrent density of 1 to 40 amps per sq. decimeter on a cathodeconsisting of cadmium, tin, bismuth, amalgamated lead, amalgamatedcadmium, amalgamated tin, amalgamated thallium, amalgamated bismuth oran alloy of at least two of the metals lead, mercury, silver, cadmium,tin, thallium and bismuth or an amalgamated alloy of the said metals.

2. A process as claimed in claim 1 in which o-phthalic acid orterephthalic acid is used in a concentration of from 2 to 8% by weightdispersed in aqueous sulfuric acid of a concentration of 2 to 50% 'byweight.

3. A process as claimed in claim 1 in which the current density is 3 to20 amps per sq. decimeter.

4. A process as claimed in claim 1 in which sulfuric acid of aconcentration of 3 to 20% by weight is used.

5. A process as claimed in claim 1 in which o-phthalic acid orterephthalic acid is used in a concentration of 3 to 8% by weight.

References Cited UNITED STATES PATENTS 2,477,579 8/1949 Condit 204-732,477,580 8/ 1949 Condit 20473 2,537,304 1/1951 Condit 204-73 PATRICK P.GARVIN, Primary Examiner mg UNITED STATES PATENT OFFICE CERTIFICATE OFCORRECTION Patent No. 3 7 ,3 1 Dated October 7, 1969 Invent fl HubertSuter et a1 It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

Column 5, line 41, "50:50" should read "50:30:20".

Column 5, in the table, example 25, under "Cathode", "51:10" should read"50:10".

when Ana 9mm FEB I 71970 5 Alton:

wmxu 1:. sown-m. Ell'lld M. Fletcher, Jr. M85193 of P3180158 Officer

